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### Deriving Identities

```
Date: 08/26/98 at 09:09:06
From: Marie McLean
Subject: Deriving Identities

Hi,

I hope you can help me with a problem I can't seem to figure out.

I'm supposed to derive the identity for sin 3x in terms of sin x.

I was told to use the identity for sin(a+b) on sin(2x + x) as a first
step. I tried that and I was able to get further but I keep doing
something wrong.

I would appreciate any help.
```

```
Date: 08/26/98 at 11:30:14
From: Doctor Rick
Subject: Re: Deriving Identities

Hi, Marie -

How far were you able to get? How do you know you did something wrong
- do you know what the answer should be? If so, it's possible your
answer is correct but that you need to apply a trig identity to get it
in the form you expect.

If you write the formula for sin(2x + x) and substitute the formulas
for sin(2x) and cos(2x) where the latter appear in the formula, you
should get an expression that has sin(x) and cos(x). You can eliminate
the cos(x) and get it in a nice form by using cos^2(x) = 1 - sin^2(x).

- Doctor Rick, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Trigonometry

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